The electronics, display and energy industries rely on the formation of coatings and patterns of conductive and other electronically active materials to form circuits on organic and inorganic substrates. The primary methods for generating these patterns are screen printing for features larger than about 100 μm and thin film and etching methods for features smaller than about 100 μm. Other subtractive methods to attain fine feature sizes include the use of photo-patternable pastes and laser trimming.
It is the trend in the electronics industry to make smaller and less expensive electronic devices that provide higher resolution and enhanced display performance. As a result, it has become necessary to develop new materials and new approaches to manufacture such devices.
Photo-patterning technologies offer uniform finer lines and space resolution when compared to traditional screen-printing methods. A photo-patterning method, such as DuPont's FODEL@ printing system, utilizes a photoimageable organic medium as found in U.S. Pat. No. 4,912,019; U.S. Pat. No. 4,925,771; and U.S. Pat. No. 5,049,480, whereby the substrate is first completely covered (printed, sprayed, coated or laminated) with the photoimageable thick film composition and dried. An image of the circuit pattern is generated by exposure of the photoimageable thick film composition with actinic radiation through a photomask bearing a circuit pattern. Actinic radiation is radiation such as ultraviolet that may cause photochemical reactions. The exposed substrate is then developed. The unexposed portion of the circuit pattern is washed away leaving the photoimaged thick film composition on the substrate that, subsequently, is fired to remove all remaining organic materials and sinter inorganic materials. Such a photo-patterning method demonstrates line resolution of about 30 microns depending on the substrate smoothness, inorganic particle size distribution, exposure, and development variables. When employed for the production of conductors in display devices such as plasma display panels, field emission displays, or liquid crystal displays, the conducting lines can be up to a meter long, many orders of magnitude longer than their widths and precision. The process is necessarily subtractive in its nature as a result of the wash-out of a large portion of the pattern. A process that is additive is desired by those in the industry.
The ink jet printing system is a high resolution, additive printing system having the ability to print complex patterns through digital instructions. This ink jet printing system is a recording system, which prints by discharging ink drops through a discharge orifice such as a nozzle or a slit to thus make the ink drops directly adhere to a printing substrate. Ink jet techniques usually fall into two broad categories: continuous injection systems and on-demand systems. In continuous injection systems, the ink jet is firing a continuous stream of microdrops and the pattern is established by selectively diverting or not diverting those microdrops to a waste reservoir. This system cannot be viewed as fully additive in that the portion of material diverted to the reservoir is lost, making the process less than 100% additive. In the on-demand system, drops are fired only when required. These systems are more prone to clogging when employing inks with high solids content, and it is a common feature that the first several drops on demand may not fire.
The ink mainly used in such an ink jet printing system comprises a dye dissolved in an aqueous or nonaqueous solvent. In the field of conductive inks, a liquid dispersion of ultrafine metal particles has been used in the formation of a conductive circuit making use of the ink jet printing system (US patent application 2003/0110978 A1). Liquid dispersions of other ultrafine particles such as metal oxides, organometallics or polymers may also be used in the formation of components of electronic circuits or display devices using ink jet printing systems.
United States patent application US 2004 (009290) discloses the spinning of conductive fibers or ribbons that are attached to a substrate by spinning a fiber or ribbon composed of an organic polymer with an inorganic material and affixing that fiber or ribbon in a desired orientation on a substrate and finally heating the composition to remove the organic polymer. This results in the conductive fiber or ribbon being affixed to the substrate in the desired orientation. Suitable inorganic materials are generally metal conductors that include Au, Ni, Au—Cr alloy, Au—Ta alloy, Cu—Cr alloy, Au/Indium tin oxide, Cu, Ag, and Ni. These constructions are useful as electrodes particularly on silicon wafers in solar cell fabrication. While several spinning methods are discussed, the concept of utilizing a viscoelastic system in which the concentration of the polymer component is no higher than a few percent is not disclosed. United States patent application US 2004050476 is similar but directed to a process for the fabrication of features on a display panel utilizing fibers or ribbons comprising organic polymers and inorganic material, the inorganic materials being phosphors, conductive metals or dielectric particles. These applications do not anticipate the advantage of using a viscoelastic polymer solution having a low polymer content thereby maximizing the quantity of functional phase materials printed onto the substrate.
It is possible to disperse solids in many synthetic polymers and spin fibers of those polymers. This is practiced with carbon blacks (U.S. Pat. No. 4,129,677, U.S. Pat. No. 4,388,370, and EP 250664), zinc oxide (U.S. Pat. No. 5,391,432), magnesium oxide (JP 57161115), or antimony tin oxide-coated Ti oxide particles (JP 59047474) in nylon for antistatic purpose, both throughout the fiber and in segregated into the core of core-shell compositions. Tin oxide (JP 49034550) has been added as a flame retardant. In all of these examples, the polymer component is an appreciable fraction and usually the majority of the mixture being spun. Conductivities of the resulting system are relatively low because the content of the active phase is necessarily so low. In addition, the fibers would have to be adhered to the substrate surface at very high temperatures for them to adhere. To achieve higher conductivities, the polymer fraction would have to be fired out, but the polymer content is so high that the volatilization process would destroy the lines.
An advantage of the composition in the current invention is that the polymer content is lower than the content of the functional phase and that it may be spun and adhered to the substrate surface at conditions close to ambient.
Various methods for preparing the foregoing liquid dispersion of metal ultrafine particles are known. The metal ultrafine particles or powder can be dispersed together with, for instance, a solvent, a resin and a dispersant, according to various means such as stirring, the applying of ultrasonic waves and mixing in, for instance, a ball mill or a sand mill. Liquid dispersions prepared according to this method have been employed in the fields of inks, paints and varnishes. There have been known, for instance, a method for directly preparing metal ultrafine particles in a liquid phase such as the technique for the evaporation of a metal in a gas phase (subsequently referred to as “evaporation-in-gas” technique) comprising the steps of evaporating a metal in a low vacuum atmosphere in the coexistence of vapor of a solvent, and then condensing the evaporated metal and solvent into uniform metal ultrafine particles to thus disperse the resulting particles in the condensed solvent and to thus give a liquid dispersion (Japanese Patent No. 2,561,537) and those, which make use of an insoluble precipitate-forming reaction or a reducing reaction using a reducing agent. Among these methods for preparing liquid dispersions of metal ultra fine particles, the evaporation-in-gas technique would permit the stable preparation of a liquid dispersion containing metal ultrafine particles having a particle size of not more than 100 nm and which are uniformly dispersed therein. In the evaporation in-gas technique, the amount of a dispersion stabilizer or a component required for the preparation of a liquid dispersion containing metal ultrafine particles in a desired concentration is smaller than that required in the liquid phase preparation technique.
Dispersion of metal or other ultrafine particles must have characteristic properties (such as viscosity and surface tension) required for the ink used in ink jet printing. Ultrafine particles often undergo aggregation and this makes it difficult to prepare any liquid dispersion in which the ultrafine particles are dispersed in a stable manner. For this reason, when using such a liquid dispersion of ultrafine particles as ink for the inkjet printing, the ink suffers from a problem in that aggregates of the ultrafine particles present therein result in clogging of the ink jet nozzles. Moreover, when using a liquid dispersion in which the ultrafine particles are independently or separately dispersed, as ink for the ink jet printing, the liquid dispersion should be prepared using a solvent suitable for satisfying the requirements for characteristic properties of the ink. However, the choice of a solvent suitably used in the ink for the ink jet printing has been quite difficult. The properties required for an ink jet system are often at odds with the requirements for stable dispersions of the ultrafine particles.
Ink jet techniques necessarily require low viscosity fluids for proper operation of the jetting system. It is difficult to build features to any appreciable thickness, though this can be done utilizing multiple passes. Drying time or some other means for stabilizing the initial feature is required between passes. Resolution is often compromised and it is difficult to obtain appreciable feature height to feature width because non-viscous, wetting fluids are employed.
Despite the foregoing advances in such systems, manufacturers are continuously seeking compositions with improved utility of the ultrafine materials and finer resolution of lines and spaces. Such materials will increase the speed of the manufacturing processes without compromising high resolutions in the lines and spaces of the circuit or display patterns. The present invention is directed to such a process, the materials and compositions required for implementation of the process, and the methods for production of said materials.
In solution spinning, a concentrated solution of a polymer is forced through a spinneret. The face of the spinneret is in contact only with a gas, which is usually air. Because solvent evaporation is generally a slow process, after travelling a short distance through the air, typically 0.1-10 cm., the concentrated solution (in the form of a fine “jet”) usually enters a coagulant, which extracts the solvent from the polymer, resulting in the formation of a polymer fiber. The coagulant is frequently water or, as in the case of the present invention, air. Importantly, in the gap between the spinneret face and the coagulant, the fiber solution, which is usually quite viscous and somewhat viscoelastic, is drawn, resulting in a smaller diameter jet of polymer solution entering the coagulant than was extruded from the spinneret holes. The amount of drawing that can be done is limited, because above some maximum draw value the fibers tend to break.